This application is based on Japanese patent applications No. 10-69454 filed on Mar. 4, 1998, and No. 10-150730 filed on May 14, 1998, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a solid state image pickup device capable of storing an increased amount of signal charges in each light reception element, the solid state image pickup device being of the type that signal charges stored in light reception elements are read to form a frame image through interlace drive.
b) Description of the Related Art
A charge coupled solid state image pickup device (hereinafter called a CCD image pickup device) of an interline transfer type is known. As shown in FIG. 7A, this CCD image pickup device is constituted of: a number of photodiodes (light reception elements) PD1,1 , to PDI,J disposed in a matrix (rows and columns) layout; vertical transfer paths VCT1 to VCTJ juxtaposed with the photodiodes PD1,1 to PDI,J via transfer gates TG1,1 to TGIj; a horizontal transfer path HCT connected at one ends of the vertical transfer paths VCT1 to VCTJ; and an output circuit OUT connected to at one end of the horizontal transfer path HCT.
The vertical transfer paths VCT1 to VCTJ vertically transfer signal charges photo-excited and stored in the photodiodes PD1,1 to PDI,J in response to four-phase drive signals V1 to V4 synchronizing with a signal representative of one horizontal scan period. The horizontal transfer path HCT horizontally transfers the signal charges transferred from the vertical transfer paths VCT1 to VCTJ in response to two-phase drive signals H1 and H2 synchronizing with a signal representative of one horizontal blanking period. By repeating such vertical and horizontal transfer operations, the output circuit OUT outputs a pixel signal Vout of one frame.
Of the photodiodes PD1,1 to PDI,J, those on odd rows are assigned an odd field, and those on even rows are assigned an even field. With an interlace drive using a vertical transfer by the four-phase drive signals V1 to V4 and a horizontal transfer by the two-phase drive signals H1 and H2, two fields of an odd field and an even field are read to output the pixel signal Vout of one frame.
The interlace drive is executed synchronously with four-phase drive signals V1 to V4 such as shown in FIG. 7B. Potential profiles of the transfer gates TG1,1 to TGI,J and vertical transfer paths VCT1 to VCTJ change synchronously with the four-phase drive signals V1 to V4, as shown in FIGS. 8AA to 8EB, in order to read odd and even fields.
In reading signal charges QO of the odd field shown hatched, the potential profiles change as shown in FIGS. 8AA, 8BA, 8CA, 8DA, and 8EA. In reading signal charges QE of the even field shown hatched, the potential profiles change as shown in FIGS. 8AB, 8BB, 8CB, 8DB, and 8EB. In FIGS. 8AA to 8EB, the abscissa represents a horizontal direction in FIG. 7A and the ordinate represents a potential. The photodiodes and transfer gates of the odd field are represented by PDO and TGO, respectively. The photodiodes and transfer gates of the even field are represented by PDE and TGE, respectively. Each or all of the vertical transfer paths VCT1 to VCTJ are represented by VCT where applicable.
During an exposure period xcfx84ON shown in FIG. 7B, all channel potentials (hereinafter called channel barriers) under the transfer gates TGO and TGE are made high as shown in FIGS. 8AA and 8AB so that photo-excited signal charges QO and QE corresponding in amount to an object illuminance hxcexd are stored in all photodiodes PDO and PDE.
At a timing t1 after the exposure period xcfx84ON, the drive signal V1 which is a pulse PL1 having a level higher than a predetermined threshold voltage Vth is applied to the transfer gates TGO of the odd field. Therefore, as shown in FIG. 8BA, only the channel barriers under the transfer gates TGO are made low so that only the signal charges QO of the photodiodes PDO are transferred to the vertical transfer paths VCT. As shown in FIG. 8BB, the signal charges QE in the photodiodes PDE are not transferred to the vertical transfer paths VCT but are stored in the photodiodes PDE.
During an odd field read period xcfx841 (FIG. 7B), the vertical transfer paths VCT1 to VCTJ vertically transfer the transferred signal charges QO in synchronization with the four-phase drive signals V1 to V4, and the horizontal transfer path HCT horizontally transfers the vertically transferred signal charges QO and outputs the pixel signal Vout corresponding to the odd field. The potential profiles during the odd field read period xcfx841 are as shown in FIGS. 8CA and 8CB. The potential profile shown in FIG. 8CB will be later detailed.
At a timing t2 after the odd field read period xcfx841, the drive signal V3 which is a pulse PL3 having a level higher than the predetermined threshold voltage Vth is applied to the transfer gates TGE of the even field. Therefore, as shown in FIGS. 8DA and 8DB, only the channel barriers under the transfer gates TGE are made low so that only the signal charges QE of the photodiodes PDE are transferred to the vertical transfer paths VCT.
During an even field read period xcfx842, the vertical transfer paths VCT1 to VCTJ vertically transfer the transferred signal charges QE in synchronization with the four-phase drive signals V1 to V4, and the horizontal transfer path HCT horizontally transfers the vertically transferred signal charges QE and outputs the pixel signal Vout corresponding to the even field. The potential profiles during the even field read period xcfx842, are as shown in FIGS. 8EA and 8EB.
As above, the pixel signals of one frame can be read by reading two fields during the odd field read period xcfx841 and even field read periods xcfx842.
The conventional CCD image pickup device is, however, associated with some problem which is caused by reading the pixel signals of one frame from two fields, at different timings.
With th conventional CCD image pickup device, the signal charges in the photodiodes PDE of the even field are read during the even field read period xcfx842 after the odd field read period xcfx841. However, as shown in FIG. 8CB, during the odd field read period xcfx841, the signal charges QE leak to the semiconductor substrate so that the signal charge amount in the photodiode PDE reduces more than the signal charge amount photo-excited and stored therein the exposure period xcfx84ON.
This leak phenomenon may be ascribed to thermal emission of signal charges in the photodiode PDE, because of the thermal emission, the signal charges passing over the potential barrier between the photodiode PDE and semiconductor substrate and flowing toward the semiconductor substrate. A current I converted from the charge amount leaked from the photodiode PDE to the semiconductor substrate is theoretically expressed by:
Ixe2x88x9d exp(xe2x88x92qVbar/kT)
where Vbar is a potential of the potential barrier, q is a signal charge, k is the Voltzmann""s constant, and T is an absolute temperature.
The signal charges in the photodiode PDO are rarely subject to the leak phenomenon, because they are read first during the odd field read period xcfx841. On the other hand, the signal charges in the photodiode PDE are influenced by the leak phenomenon, because they are stored until the odd field read period xcfx841 finishes and they are leaked during this store period.
Therefore, an amount (hereinafter called a saturated charge amount) of signal charges capable of being stored in the photodiode PDO of the odd field becomes substantially different from a saturated charge amount of signal charges capable of being stored in the photodiode PDE of the even field, resulting in a difficulty of forming a clear frame image. This problem becomes conspicuous when a still image is formed.
A general movie camera having a CCD image pickup device continuously repeats exposure and signal charge read. Therefore, photodiodes of both the odd and even fields operate under the same exposure and signal read conditions. The saturated charge amounts of the odd and even fields do not therefore become different, and the problem of different saturated charge amount between odd and even fields will not occur.
However, in an electronic still camera having a shutter for forming a still image, the shutter is closed after exposure and signal charges of the odd and even fields are read at different timings under the condition of no exposure light. Therefore, during the odd field read period before the even field read period, the amount of signal charges of the even field under a standby state reduces because of the leak phenomenon. There arises therefore the problem of different saturated charge amounts between odd and even fields.
As above, the influence of the leak phenomenon poses a significant problem for a CCD image pickup device of the type that fields are read under the condition of shielded incidence light or no incidence light.
Next, an overflow drain will be described. A transfer gate is provided between each photodiode and an adjacent vertical transfer path. By controlling the level of the channel barrier under the transfer gate, signal charges photo-excited and stored in the photodiode can be transferred to the vertical transfer path.
Specifically, signal charges corresponding in amount to an object illuminance can be photo-excited and stored in each photodiode, by raising the channel barrier under the transfer gate. In reading the signal charges after the exposure, the channel barrier under the transfer gate is lowered once to transfer the signal charges stored in the photodiode, and then the channel barrier is again raised. In this manner, the signal charges are transferred by the vertical and horizontal transfer paths.
A so-called sensor blooming phenomenon may occur if only the channel barrier under the transfer gate is controlled. As a countermeasure for this phenomenon, an overflow drain is formed by using a potential barrier different from that under the transfer gate.
If the overflow drain is not formed, excessive charges generated upon incidence of strong (excessive) light upon a photodiode pass over the channel barrier under the transfer gate and leak into the vertical transfer path. This phenomenon is called a sensor blooming phenomenon. If an image is reproduced from image signals read under such conditions, vertical stripes appear on the reproduced image and the image quality is lowered.
In order to avoid this, the overflow drain is formed adjacent to each photodiode. During exposure, the level HOFD of a potential barrier between the overflow drain and photodiode is set lower than the level HTG of the channel barrier under the transfer gate, so that excessive charges are flowed toward the overflow drain in order not to leak the excessive charges to the vertical transfer path.
A conventional CCD image pickup device of an interline transfer type is, however, associated with some problem if exposure is performed under the condition that the level HOFD of a potential barrier between the overflow drain and photodiode is set lower than the level HTG of the channel barrier under the transfer gate.
Specifically, although it is effective for preventing occurrence of the sensor blooming phenomenon to perform the exposure satisfying a relation of HTG greater than HOFD between the channel barrier and potential barrier, signal charges having the level higher than a difference between the potential barrier level HOFD and the channel barrier level HTG are always drained to the overflow drain. Therefore, the saturated charge amount of the photodiode reduces by an amount corresponding to a barrier level difference |HTGxe2x88x92HOFD|. It is therefore difficult to manufacture a CCD image pickup device having excellent performances such as wide dynamic range with high sensitivity.
It is an object of the present invention to provide a solid state image pickup device capable of uniformalizing saturated charge amounts of all light reception elements.

It is another object of the present invention to provide a solid state image pickup device capable of providing all light reception elements with an increased saturated charge amount.
According to one aspect of the present invention, there is provided a method of controlling a solid state image pickup device of an interlace type the solid state image pickup device including a semiconductor substrate formed with at least: light reception elements disposed in two-dimensional rows and columns for performing charge generation and accumulation through photoelectric conversion, the light reception elements being assigned to first and second fields; a plurality of vertical charge transfer paths for transferring the charges generated by the light reception elements, each vertical transfer path being provided in correspondence with each light reception column; overflow drain regions capable of draining charges in the light reception elements; a first semiconductor region between each light reception element and a corresponding vertical charge transfer path; a second semiconductor region between each light reception element and a corresponding overflow drain region; and a transfer gate provided for each light reception element for transferring charges in each light reception element to a corresponding vertical charge transfer path by controlling a potential of the first semiconductor region, the transfer gate being formed over the first semiconductor region with an insulating film being interposed therebetween, and the method comprising the steps of: (a) generating and accumulating charges in each light reception element in the first and second fields, by setting a potential of the second semiconductor region to a first potential and by setting a potential of the first semiconductor region higher than the first potential; (b) transferring the charges in each light reception element in the first field to the vertical charge transfer paths, by lowering the potential of the first semiconductor region corresponding to the light reception element in the first field; (c) transferring the charges corresponding to the first field in the vertical charge transfer paths, by setting a potential of the second semiconductor region higher than a second potential higher than the first potential; (d) transferring the charges in each light reception element in the second field to the vertical charge transfer paths, by lowering the potential of the first semiconductor region corresponding to the light reception element in the second field; and (c) transferring the charges corresponding to the first field in the vertical charge transfer paths.
The potential of the second semiconductor region between each light reception element and a corresponding overflow drain is set higher than that during charge generation in each light reception element, at least during a first field read operation. Therefore, until the second field read operation starts, it becomes difficult for signal charges stored in the light reception elements to pass over, because of thermal emission of charges, the potential barrier of the second semiconductor region and leak to the overflow drain region during the first field read operation. The saturated charge amounts of the light reception elements in the first and second fields can therefore be uniformalized.
The solid state image pickup device may comprises light interception/transmission means for switching between interception and transmission of incidence light to the light reception elements, wherein: the step (b) generates charges in the light reception elements in a state that the incidence light to the light reception elements is transmitted; and the step (c) transfers the charges in a state that the incidence light to the light reception elements is intercepted.
Charges of the light reception elements of the first field are transferred in the state that incidence light to the light reception elements is intercepted by the light transmission/interception means. Accordingly, until the second field read operation starts, signal charges stored in the light reception elements are hard to pass over the potential barrier of the second semiconductor region and leak to the overflow drain region because of thermal emission of the signal charges. The saturated charge amounts of the light reception elements in the first and second fields can therefore be uniformalized.
Since the saturated charge amounts of all light reception elements can be uniformalized, a frame image reproduced from image signals obtained by the first and second field read operations has less variation in luminance and has high quality.
By applying a solid state image pickup device of the invention to an electronic still camera or the like provided with light transmission/interception means, a frame image having higher quality than a conventional camera can be obtained.